function formHandler(){
var URL = document.form.site.options[document.form.site.selectedIndex].value;
window.location.href = URL;
}

document.write('<form name="form">');
document.write('<select name="site" size=1>');
document.write('<option value="home.htm">Libros selectos para ver los detalles');
document.write('<option value="manual.htm">Manual De Formulaciones De Preparaciones Farmaceuticas');
document.write('<option value="indus.htm">Industria Productos Herbaria');
document.write('</select>');
document.write('<input type=button value="Go!" onClick="javascript:formHandler()">');
document.write('</form>');